Player > Feats > Step Up and Strike
When a foe tries to move away, you can follow and make an attack of opportunity.
Dex 13, Step Up, base attack bonus +6.
When using the Step Up feat to follow an adjacent foe, you can move up to 10 feet. You can also either make an attack of opportunity against the foe, or wait to see if the foe provokes another attack of opportunity at any point before the end of its turn. Either way, this attack of opportunity does not count toward the number of actions you can usually take each round; it is part of the Step Up reaction.